1. Field of the Invention
The present invention relates to a differential signal transmission cable, more particularly, to a differential signal transmission cable for transmitting high speed digital signals corresponding to 10 Gbps or more over a distance of several meters to several tens of meters with less signal waveform distortion.
2. Related Art
In servers, routers and storage associated equipments for processing high speed digital signals of several Gbps or more, differential signal transmission is used for signal transmission between devices or between boards in the same device, and a differential signal transmission cable is used for electrical connection therebetween.
The “differential signal transmission” is a signal transmission of transmitting two kinds of signals, in which a phase of one signal is inverted by 180 degrees from a phase of another signal, through a pair of two-conductor wires respectively, and by synthesizing and outputting a difference between the two signals at a receiving end side. Since electric current flown through one of the two-conductor wires and electric current flown through another one of the two-conductor wires are flown in directions opposite to each other, an electromagnetic wave radiated from the differential signal transmission cable which serves as a transmission line is small. Further, since extraneous noises equally superpose on the two-conductor wires, the extraneous noises are canceled (offset) by synthesizing and outputting the difference at the receiving end side, so that adverse influences by the extraneous noise can be eliminated. For these reasons, the differential signal transmission has been often used for high speed signals.
As a differential signal transmission cable, a twisted-pair cable has been known. In the twisted-pair cable, two insulated electric wires each of which has a conductor wire coated with an insulating member are twisted as one pair. The twisted-pair cable is inexpensive and excellent in balancing characteristics. Further, the twisted-pair cable can be easily bent. Therefore, the twisted-pair cable has been used broadly for a mid-distance signal transmission. However, since the twisted-pair cable has no conductor corresponding to a ground, the twisted-pair cable is easily affected by a metal member located in vicinity of the twisted-pair cable, so that characteristic impedance of the twisted-pair cable is not stable. Further, in the twisted-pair cable, a signal waveform is easily distorted in a high frequency band of several GHz. Therefore, it is difficult to employ the twisted-pair cable for the high speed signal transmission of several Gbps or more.
On the other hand, a so-called “twinax cable” in which two insulated electric wires are disposed in parallel without being twisted, and coated with a shield conductor has been used. The “twinax cable” is also called as “twin-axial cable” or “twin coaxial cable”. In the twinax cable, the two insulated electric wires are disposed in parallel without being twisted, so that there is little difference in physical length between the two-conductor wires, compared with the twisted-pair cable. In addition, since the shield conductor is disposed to cover the two insulated electric wires, even if the metal member is installed in vicinity of the twinax cable, the characteristic impedance of the twinax cable will not become unstable, and the noise resistant property is high. The twinax cable has been used for the signal transmission at a relatively high speed over a short distance. In the twinax cable, for example, a tape with a metal foil (i.e. a metal foil composite tape), a braided wire and the like are used as a shield layer. A drain wire or the like may be provided together with the shield layer.
For example, FIG. 3 shows an example of conventional twinax cables disclosed by Japanese Patent Laid-Open No. 2002-289047 (JP-A 2002-289047). Referring to FIG. 3, in a conventional differential signal transmission cable 31, two signal transmission conductor wires 32, 32 are insulated by insulating members 33, 33, respectively to provide two insulated electric wires 34, 34, a drain wire 35 is lengthwise provided, a metal foil composite tape 38 in which a metal foil 37 is adhered to a plastic tape (plastic film) 36 is spirally wound around the two insulated electric wires 34, 34 and the drain wire 35, and a jacket 39 is provided to jacket the periphery of the metal foil composite tape 38 in order to protect the inside.
FIGS. 4A and 4B show another example of conventional twinax cables disclosed by Japanese Patent Laid-Open No. 2002-289047 (JP-A 2002-289047), similarly to the differential signal transmission cable 31. Referring to FIGS. 4A and 4B, in a conventional differential signal transmission cable 41, two signal transmission conductor wires 42, 42 are insulated by insulating members 43, 43, respectively to provide two insulated electric wires 44, 44, a metal foil composite tape 47 in which a metal foil 46 is adhered to a plastic tape 45 is lengthwise wrapped (in a manner of cigarette-wrapping) around the two insulated electric wires 44, 44 A drain wire 48 is lengthwise provided between the metal foil composite tape 47 and the insulated electric wires 44, 44 to contact a conducting plane (the metal foil 46) of the metal foil composite tape 47, to be grounded. An outer surface of the metal foil composite tape 47 is jacketed with a jacket 49 so as to protect the inside.